Ellipsometers are optical metrology devices that detect changes in the polarization state of light reflected from a surface of a sample in order to measure characteristics of the sample. By way of example, FIG. 1 illustrates a conventional spectroscopic ellipsometer 10 that includes a broad band light source 12, a polarizer 14 and lens 15 to focus the illuminating light on the surface of a sample 16 that is positioned on a stage 18. The ellipsometer 10 further includes an analyzer 22 after passing through lens 20. After passing through analyzer 22, the reflected light is focused by lens system 24 on a detector 26.
The ellipsometer 10 must be properly focused on the sample. Some systems use independent focusing systems, i.e., systems that attached to the ellipsometer, but that use an independent light path to determine the position of the focusing system, and thus, the ellipsometer, with respect to the sample. Such focusing systems, however, require very precise alignment, which is expensive and difficult. FIG. 1 illustrates example of an integrated focusing system that includes a mirror 28 with an aperture 30. The mirror 28 reflects the outside rays of the reflected beam to a focus detector 32, while the inner rays of the reflected beam are transmitted through the aperture 30 and received by the ellipsometer detector 26. Typically, the focus detector 32 is a position sensitive device (PSD) or a “quad cell”, neither of which image the detected light but, in principle, sums all of the light that is received. Accordingly, such devices are susceptible to inaccuracies due to stray light. Other systems, such as that described in U.S. Pat. No. 5,608,526, use a camera as the focus detector 32. Nevertheless, such a system still suffers from imprecision due to systematic errors caused by the mirror 28, because only the outer part of the beam is sampled by detector 32. Moreover, a focus system, such as that described in U.S. Pat. No. 5,608,526, lack the precision required to produce a small illuminating spot size on the sample.
Accordingly, an improved focusing system for ellipsometers is desired.